Liquid crystals have found use in a variety of electrooptical and display device applications, in particular those which require compact, energy-efficient, voltage-controlled light valves such as watch and calculator displays. Liquid crystal displays have a number of unique characteristics, including low voltage and low power of operation, which make them perhaps the most promising candidate materials for non-emissive electrooptical displays available with current technology. Most of these devices are based upon the dielectric alignment effects in nematic, cholesteric and smectic phases of the liquid crystal compound in which, by virtue of dielectric anisotropy, the average molecular long axis of the liquid crystal molecules take up a preferred orientation in an applied electric field. Since the coupling to the applied electric field by this mechanism is rather weak, the resultant electrooptical response may be too slow for many potential applications. The requirement for speed may become especially important in proportion to the number of elements which have to be addressed in a device. This may result in increasingly impractical production costs for the potential use of such devices in flat-panel displays for use in video terminals, oscilloscopes, radar and television screens.
Twisted nematic (TN) liquid crystals are currently widely employed for display applications. TN devices and related super twist nematic (STN) devices require nematic liquid crystal compositions with positive dielectric anisotropy. Nematic LC device applications require chemically stable LC compounds or mixtures of compounds which display a nematic phase over a substantial temperature range, preferably about room temperature. Oftentimes, desired electrooptic properties can be achieved in a nematic liquid crystal by addition of dopants to nematic host materials. The kind and amount of such dopants added allows the tuning of electrooptic properties in the resultant mixtures. LC dopants which enhance or introduce desired electrooptic properties in LC mixtures without detriment to mesomorphic properties are important for the production of LC compositions for device applications. Nematic LC compositions having high birefringence are important for certain high contrast display applications. LC dopants which enhance birefringence in LC mixtures without detriment to mesomorphic properties are important for such high contrast applications.
Passive matrix STN liquid crystal displays are widely used in flat panel applications. Scheffer, T.J. and Nehring, J. (1984) Appl. Phys. Lett. 45:1021. STN response times are sufficiently fast to follow cursor movement but remain too slow for video applications. The standard multiplexed addressing method used to drive these displays appears to limit speed. Using a recently developed drive method called active addressing, bright, high contrast, full color, full motion video images have been demonstrated on STN displays. Scheffer, T.J. and Clifton, B. (1992) SID 92:228-231; Conner, A.R. and Scheffer, T.J. (1992) Japan Display 92:69; Clifton, B. et al. (1992) Japan Display 92:503. Active addressing STN (ASTN) was demonstrated only at elevated temperatures, which although useful in projection displays is not useful for other display applications including those for lap-top computers. To produce full motion video images with active addressing, an inherent STN response of 50 ms or faster is needed. STN cell response is inversely related to the square of the cell thickness, thicker cells having slower response times. Reductions of about 15% in cell thickness are required with the best available STN's to achieve the response time needed for video applications at ambient temperatures. However, contrast generally decreases as cell thickness decreases. To preserve contrast while reducing cell thickness for speed enhancement, the birefringence of the STN liquid crystal must be increased. Furthermore, increased birefringence must be obtained without significant increase in viscosity of the liquid crystal.
LC dopants can be employed to achieve desirable properties in STN liquid crystal compositions. Dopants useful in STN compositions for ASTN applications must be compatible with and soluble in STN compositions. STN-compatible dopants minimize suppression of the I to N transition without adversely affecting the freezing point of the mixture. Preferred dopants remain soluble in host mixtures at relatively high concentrations, i.e. mixtures remain homogenous, over the temperature range of intended operation of the display. Dopants can be used to increase birefringence or dielectric anisotropy of mixtures. High birefringence materials often are unstable to UV light decomposing relatively rapidly on exposure. While UV filters can be employed in display to minimize exposure, preferred dopants are more stable to UV light. Other properties of STN mixtures that can be affected by dopants, including splay and bend elastic constants, as are understood in the art, may influence use of mixtures for ASTN applications. STN compositions having response times of about 50 ms or faster, birefringence (.DELTA.n) of about 0.24 or more, positive dielectric anisotropy of about 10 or more and low viscosity, most preferably 22 mm.sup.2 /sec or less, are preferred for ASTN applications.
Electrooptic effects with sub-microsecond switching speeds can be achieved using the technology of ferroelectric liquid crystals (FLCs) of N.A. Clark and S.T. Lagerwall (1980) Appl. Phys. Lett. 36:899 and U.S. Pat. No. 4,367,924. These investigators have reported display structures prepared using FLC materials having not only high speed response (about 1,000 times faster than currently used twisted nematic devices), but which also exhibit bistable, threshold sensitive switching. Such properties make FLC-based devices excellent candidates for light modulation devices including matrix-addressed light valves containing a large number of elements for passive displays of graphic and pictorial information, optical processing applications, as well as for high information content dichroic displays. A review of the applications of FLC devices is given by S.T. Lagerwall and N.A. Clark (1989) Ferroelectrics 94:3-62.
Tilted smectic liquid crystal phases, in particular smectic C phases, are useful in the preparation of FLC materials. Materials exhibiting such smectic phases which comprise chiral, nonracemic components possess a spontaneous ferroelectric polarization, or macroscopic dipole moment, deriving from a dissymmetry in the orientation of molecular dipoles in the liquid crystal phases (Myer et al. (1975) J. Phys. (Les Ulis, Fr) 36:L-69). The ferroelectric polarization density is an intrinsic property of the material making up the phase and has a magnitude and sign for a given material under a given set of conditions. In FLC devices appropriate application of an external electric field results in alignment of the molecules in the FLC phase with the applied field. When the sign of the applied field is reversed, realignment or switching of the FLC molecules occurs. This switching can be employed for light modulation. Within a wide range of electric field strengths, the switching speed (optical rise time) is inversely proportional to applied field strength and polarization or dipole density (E) and directly proportional to orientational viscosity. Faster switching speeds are thus associated with FLC phases which possess higher polarization density and lower orientational viscosity.
A basic requirement for application of ferroelectric liquid crystals in SSFLC devices is the availability of chemically stable LC compounds or mixtures which exhibit chiral tilted smectic phases, preferably chiral smectic C phases, over a substantial temperature range, preferably about room temperature. Some FLC-like devices require LC materials having a smectic A phase. In some cases, a chiral nonracemic LC material will possess an enantiotropic or monotropic chiral tilted smectic phase. FLC mixtures possessing chiral smectic phases, including those with smectic C* phases (i.e., chiral smectic C) with useful temperature ranges, can also be obtained by admixture of chiral, nonracemic compounds, designated FLC dopants, into liquid crystal host material which exhibits a desired tilted smectic phase (an FLC host material) and which may or may not be composed of chiral molecules. Addition of the FLC dopant can affect the ferroelectric polarization density and/or the viscosity of the resultant FLC mixture and thereby affect switching speed. Desirable FLC dopants are molecules which impart high ferroelectric polarization density to an FLC mixture without significantly increasing the orientational viscosity of the mixture. Components of FLC mixtures can also be adjusted to vary phase transition temperatures or ranges.
Other properties of the FLC material, for example the tilt angle of the chiral smectic phase and the birefringence of the material, can affect their usefulness for particular device applications. These properties are affected by the structures of the various components and the amounts of these components in the FLC material. Most effort in the development of FLC materials has been directed toward flat panel display applications. The optimal characteristics for FLC materials used in such displays include high spontaneous polarization (Ps) and low orientational viscosity to achieve fast switching, tilt angles of 22.5.degree. which result in maximum contrast in SSFLC cells switched between crossed polarizers, low birefringence which facilitates construction of a desirable thickness panel and broad temperature range (about room temperature). FLC materials useful in waveguides, integrated optics and spatial light modulators have somewhat different requirements. High polarization and low viscosity are desired for both display and optical switching FLC applications. Enhanced performance in optical switching FLC applications is correlated with high total refractive index changes between the switched states which is associated with high birefringence and large tilt angles. A particular type of FLC display device, a dichroic display device containing color switching elements incorporating mixtures of FLCs with dichroic dyes, also requires high tilt FLC material to achieve highest contrast. (See Ozaki et al. (1985) Jpn. J. Appl. Phys. Part I 24 (Suppl. 24-3):63-65.) For applications requiring high tilt angle and/or high birefringence it is desirable to have FLC materials which combine these properties with fast switching speed and broad room temperature smectic C* phases.
Dopants can also be employed to affect C* and N* pitch in FLC applications. Long N* pitch is associated with good alignment of FLC layers and higher contrast devices. Short C* pitch is required for Distorted Helix FLC applications. Chiral nonracemic dopants having N* pitch opposite in sign to polarization are useful as pitch compensating agents in FLC compositions. Chiral nonracemic dopants having N* pitch and C* pitch opposite in sign are useful in preparation of DHFLC mixtures having long N* pitch, preferred for alignment and high contrast.
Thermotropic liquid crystals molecules typically possess structures which combine a relatively rigid core coupled with two relatively "floppy". i.e., structurally flexible, tails. These tails are typically coupled to the core such that the LC molecule can assume a configuration with a relatively linear arrangement of the tails along the long axis of the core. Dopants useful for imparting desired properties to LC phases typically possess such a rigid core with at least one such flexible tail.
This invention provides LC compositions and LC dopants comprising a tolane core which are useful in FLC and STN applications.
U.S. Pat. No. 5,154,851 of Goto et al. relates to alkynyl tolane liquid crystal compounds having two or three-ring cores of the formula: ##STR1## wherein when n=1 (three-ring cores) A is 1,4 -phenylene, 1,4-phenylene substituted with one or two F, Cl, Br or CN, 1,4-cyclohexylene, pyrimidine-2,5-diyl or 1,3-dioxane-2,5-diyl, X is --COO--, --OCO--, --CH.sub.2 --CH.sub.2 --, --OCH.sub.2 --, --CH.sub.2 .dbd.CH.sub.2 --, or a single bond and R.sup.1 and R.sup.2 are 1-8C alkyl where one CH.sub.2 group or two non-adjacent groups may be replaced by an O, --CO--, --COO --, --OCO-- or --CH.sub.2 .dbd.CH.sub.2 and when n=0 (two-ring cores) X is a single bond with Y.dbd.H or F. These compounds are said to raise birefringence without lowering NI point and without raising viscosity.
U.S. Pat, No. 5,047,169 of Shibata et al. relates to alkynyl or alkadienyl tolanes having the formula: ##STR2## where R is an alkynyl or alkadienyl group having 3 to 18 carbon atoms and R' is an alkyl or alkoxy group having 1-18 carbon atoms. These compounds are said to elevate the anisotropy of a liquid crystal composition.
Hird and Toyne (1993) Liq. Crystals 14:741-761 relates to LC materials having high optical anisotropy. Among the compounds specifically disclosed are certain alkynyl substituted 1,4-phenyl-2,5-pyridinyl ethynes and certain alkynyl substituted 1,4-phenyl-2,5-pyrimidinyl ethynes.
A number of references refer to certain LC compounds or components having tolane or substituted tolane cores, including: Gray et al. (1989) WO 89/02425 (certain monofluorinated tolanes); German laid-open application DE 3901266 and Reiffenrath et al. (1989) Liq. Cryst. 5(1):159-170 (certain difluorinated tolanes); JP 1221352 (FLC compositions comprising certain tolanes).
Higuchi et al. U.S. Pat. No. 4,728,458 refers to chiral polyphenyl compounds useful in liquid crystal materials. The general formula (1) in the reference refers to halogenation of the core moiety which core appears to include tolanes. However, no tolanes appear to be specifically disclosed in the patent. Eidenschink et al. WO 87/05018 refers to optically active compounds and the general formula I in the reference appears to refer to halogenated tolane cores.